Oracle

Walk forward through the compound until you reach a room with copious amounts of evidence of a previous battle. Drop down to the lower level and hang out for a bit—after seeing an apparition of The Heretic, the room will be swarmed by Flood creatures that inhabit the corpses around you. Shoot them down as they stand up, and go through the doorway on the lower level as it opens up to you.

You'll walk into a room that's seeing a battle betwixt Flood and Sentinels. You can let the Sentinels clear some of the Flood, but you'll have to bust in and do the work yourself eventually. Shoot down the Sentinels first and then focus on the Flood, but make sure you don't fall off the ledges of this precarious platform.

Hang out on the platform for a while and the whole thing will start rolling downwards. Defend your position as you descend, as more Flood will jump at you from the rafters above. When the platform finally rests, watch the doorway ahead for Flood that come rolling in.

Step through the doorway and go down the next hall. In the next room you'll hear firefight below—the heretics are fighting the Flood. Stay on the upper level you're on and let them fight it out until one side is finished. Pick off the remaining Flood with shots from the Carbine, and watch for the exploding Flood creatures that release their spores (the spores will burrow into a corpse and reanimate it).

When you think the area below is clear, drop down—more enemies will spawn around you, and heretics will snipe you from the platform above (where you were previously). Use the Energy Sword if you've got it (there are a couple lying on the floor of the room) to fend off the Flood as they swarm around you, and watch as more heretics come through the doors behind the two plasma turrets.

Go through the door the heretics used when they're dead, and you'll exit to an outdoor area. Let the Covenant ship above you deal with most of the enemies below, and more fellow Covenant will drop down to aid you.

Clear the platform and run over the bridge. Run around the walkway that surrounds the next building, clearing out the heretics that cover it. On the other side (doesn't matter which direction you choose to go) is a doorway that leads into the building. Step inside and watch for the enemies filling the room—Sentinels and Flood are duking it out. Once all enemies inside are dead, a cutscene will trigger that starts the next section of the mission.

Ascend the round walkways that lead to the top of the large room, and watch for the Flood and Sentinels that greet you at the top. Use your Energy Sword to take on the Flood, and continue to the top platform. Activate the elevator to ascend to the top of the building, and ready your sword as you approach.

At the top floor are a bunch of Flood and Sentinels engaged in battle. Don't try fighting them right away—instead, run up a ramp to the upper platform on the outside of the room and take cover behind one of the pillars while dishing out the kills.

When the coast is clear, run towards the base of the cables where they connect to the walkway and slice at the backs of them with the Energy Sword. When one cable is destroyed, move to the next, and watch out for the many enemies that spawn around you. Don't concern yourself with clearing out the enemies, though—just rush to the three cable bases and disconnect them to send the building into free-fall.

Once all cables are cut, return to the center of the room and take the elevator back down. The room below is re-inhabited by heretics and Flood, and you need to reach the doorway at the bottom. Head through the doorway (the one the heretic leader hid behind earlier) and drop into the elevator shaft ahead of you. Run down to the bottom of the next room, and step through the doorway ahead.

Your stint in the Banshee will be extremely short lived. Watch out for the one attacking Banshee, and fly towards the nav-point to enter the hangar. When you land, go through the next couple of rooms infested with Flood and follow the halls clearing out the enemies within.

You'll go through numerous halls with more Flood and heretics. Eventually, after climbing numerous floors, you'll reach The Heretic. And it's time to fight him.

The battle can be either extremely simple or very difficult, depending on how you approach it. The Heretic creates two clones of himself, both of which will continue to respawn after you kill them. To defeat The Heretic you must kill the real one—which isn't too hard if you're quick.

As soon as the cutscene ends, equip your Energy Sword and rush past the two false Heretics. You'll see the real Heretic flying above you, pelting you with fire. Jump up towards him with your crosshairs firmly fixed on him, and execute one swinging blade attack to defeat the enemy.